1. Field of the Invention
The present invention relates to a compact and high-speed image forming apparatus and an image forming method.
2. Discussion of the Background
Recently, the image forming apparatus producing high-quality images having not less than 1,200 dpi has two major issues. One is to produce images at higher speeds, and the other is to become compact.
In order to produce images at higher speeds with a monochrome image forming apparatus, an electrostatic latent image bearer (hereinafter referred to as “an electrophotographic photoreceptor”, a “photoreceptor” or a “photoconductive insulator”) thereof typically has a higher linear speed and a larger diameter. A full-color image forming apparatus has two steps. The first step is to become a tandem having plural image forming elements and the following step is that electrostatic latent image bearers thereof have a higher linear speed and a larger diameter than the electrostatic latent image bearer of a monochrome image forming apparatus does. The image forming element is a minimum unit for forming images, including at least a photoreceptor, a charger, an irradiator and an image developer. Besides these, a transferer and a fixer are necessary, however, they need not be plural and may be one subject to shared use.
Basically having only one image forming element, the monochrome and single drum full-color image forming apparatuses generally have sizes dependent on the diameters of their photoreceptors. This is because members are arranged around the photoreceptor as a center in designing the image forming element. Typically, the larger the diameter of a photoreceptor, the larger the members therearound. Therefore, it is not so a serious issue to make the monochrome and single drum full-color image forming apparatuses compact.
Meanwhile, the tandem full-color image forming apparatus includes plural image forming elements (typically 4 elements) which are arranged in parallel, and has a limited minimum size even when the diameter of the photoreceptor is downsized. Therefore, the photoreceptor preferably has a diameter not greater than 40 mm. Typically, the diameter of the photoreceptor is proportional to the image forming speed, and therefore the smaller the diameter, the lower the image forming speed. Therefore, the linear speed of the photoreceptor has been increased as high as possible to increase the image forming speed.
However, the capabilities of members forming image forming elements such as a charger and an irradiator have been limited, and it has been difficult to design a compact image forming apparatus (the diameter of the photoreceptor is not greater than 40 mm), producing high-resolution images (not less than 1,200 dpi) at a high speed (not less than 50 pieces/min).
The chargeability of the charger needs to be improved to produce images at a higher speed. When the photoreceptor has a smaller diameter, a facing width (called a charging nip) between the photoreceptor and the charger right in front of each other is quite small (narrow) Chargers using a wire method, typified by scorotron chargers, can increase corona application to the surface of the photoreceptor by increasing the number of wires. However, the wires interfere with each other when too close to each other, and the electric power consumption increases. In addition, a grid is needed for charge stability and the charging nip width depends on the size thereof. Typically, the grid is formed of an electroconductive metal plate and located in the tangential direction of the photoreceptor. Therefore, when the photoreceptor has a smaller diameter, distances between the grid and the surface of the photoreceptor are largely different at the middle of the grid and both ends thereof, and the substantial nip width is very narrow (both ends of the photoreceptor are unstably charged). In order to solve this, a grid which is not a flat plate and curved in accordance with the curvature of the photoreceptor can be used. However, this is not practical because this makes the apparatus more complicated and the space for the charger is small.
There is a method of using a roller-shaped charger. The roller-shaped charger is located contacting the surface of a photoreceptor or close thereto with a gap of about 50 μm therebetween. Typically, the surfaces thereof rotate at an equivalent speed in the same direction, a bias is applied to the roller and the roller discharges to the photoreceptor to be charged. The smaller the diameter of the charger, the more compact the charger. When the charger has a small diameter, the chargeable range (a range wherein a gap between the photoreceptor and the surface of the roller; called a charging nip) becomes narrow and deteriorates in chargeability. However, the chargeability is not so deteriorated as the scorotron charger, and when a DC bias overlapped with an AC bias is applied to the roller, the chargeability noticeably improves. Therefore, the charging process is not limited if these technologies are used. However, the DC bias overlapped with an AC bias is a large stress to the surface of the photoreceptor, resulting in deterioration of durability (life) thereof.
On the other hand, light emitting diodes (LEDs) and laser diodes (LDs) have been used as a writing light source. The LEDs are located close to a photoreceptor in the longitudinal direction in the shape of an array. However, the resolution depends on the size of an element thereof and distances between the elements. Therefore, it cannot be said that the LED is most suitable for a light source of not less than 1,200 dpi at present. The LD emits a writing beam through a polygon mirror to a photoreceptor in the longitudinal direction thereof. When the photoreceptor has a small diameter, the linear speed thereof increases and the rotation number of the polygon mirror needs to be increased. However, the maximum rotation number of the polygon mirror is at present about 40,000 rpm and a single beam has a limited writing speed.
Plural light beams are beginning to be used. Plural LD light sources irradiate beams to a polygon mirror or a multibeam irradiator including plural LDs in an array is used. Recent multibeam irradiators include a surface emitting laser having three or more light sources and a surface emitting laser having two-dimensional light sources. These can write images having a resolution not less than 1,200 dpi on a photoreceptor.
Thus, with the improvements or new technologies of members forming the image forming elements, it is ready to prepare a compact image forming apparatus (the diameter of the photoreceptor is not greater than 40 mm), producing high-resolution images (not less than 1,200 dpi) at a high speed (not less than 50 pieces/min).
When the compactness and high-speed are to be realized at the same time, it is not clarified which part such as a linear speed of a photoreceptor and sizes of members therearound is a limiting factor.
The present inventors made various simulations of limiting process in the compact image forming apparatus (the diameter of the photoreceptor is not greater than 40 mm), producing high-resolution images (not less than 1,200 dpi) at a high speed (not less than 50 pieces/min). As a result, the linear speed of a photoreceptor needs to be increased when forming images at a high speed with the photoreceptor having a small diameter, however, the linear speed depends on an image forming speed set in the apparatus and a paper spacing. When the image forming speed is fixed, the shorter the paper spacing, and the lower the linear speed can be. However, the linear speed has a minimum limit as the paper spacing does.
The linear speed influences capabilities and sizes of image forming elements around the photoreceptor. As mentioned above, when the charger has sufficient chargeability, the charger can be small and the photoreceptor has an extra space therearound. Therefore, for example, a discharger and an irradiator can advantageously be relocated. When the photoreceptor does not have a sufficient potential reduction after discharge, an interval (distance) between the discharge and charge can be extended because the charger is small. Alternatively, when the photoreceptor does not have a sufficient potential reduction after irradiated, the irradiator can be located close to the charger and an interval (distance) between the irradiation and development can be extended.
In the compact image forming apparatus (the diameter of the photoreceptor is not greater than 40 mm), producing high-resolution images (not less than 1,200 dpi) at a high speed (not less than 50 pieces/min), the present inventors found that a time from the irradiation to the development (hereinafter referred to as an “irradiation-development time”) is extremely short. Specifically, the current image forming apparatuses have an irradiation-development time of about 70 msec at earliest, however, the above-mentioned image forming apparatus has an irradiation-development time less than 50 msec.
There has been no photoreceptor used in an image forming apparatus having such a short irradiation-development time. The present inventors evaluated a time-responsiveness of surface potential light attenuation of a photoreceptor to search properties of a photoreceptor usable therein.
As a method of evaluating the time-responsiveness of surface potential light attenuation of a photoreceptor, Published Unexamined Japanese Patent Applications Nos. 10-115944 and 2001-312077 disclose a Time of Flight (TOF) method of evaluating a resin layer including a charge transport material (CTM) or a CTM and a binder resin. This is effectively used to design the formulation of a photoreceptor. However, there is a difference between the charge transport conditions of a photoreceptor used in an apparatus and those of a TOF method, i.e., an electrical field intensity in the layer of the former photoreceptor momentarily changes, and that in the layer of the latter photoreceptor is constant. In addition, the TOF method does not reflect a charge generation from a charge generation layer (CGL) and a charge injection therefrom to a charge transport layer (CTL) of a multilayer photoreceptor.
As a method of directly measuring the responsiveness of a photoreceptor, Published Unexamined Japanese Patent Application No. 2000-305289 discloses a method of recording the surface potential variation of a photoreceptor after irradiation with pulse light at a high speed with a high-speed surface potential meter; and measuring a response time required for having a predetermined potential. This is typically called a Xerographic Time of Flight (XTOF) method, and is effectively used to resolve the disadvantage of the TOF method. However, most of the light sources used in this method are different from irradiators used in electrophotographic image forming apparatuses, and the method cannot exactly be considered a direct measuring method.
Published Unexamined Japanese Patent Application No. 2000-275872 discloses a measurer measuring properties of a photoreceptor, which can fix a predetermined time (hereinafter referred to as an “irradiation-development time”) for an irradiated part of the photoreceptor to reach an image developer and let a relationship (light attenuation curve) between a light quantity (energy) from a LD and an irradiated part potential be known. An embodiment of the relationship is shown in FIG. 2. FIG. 2 shows that there is an area where the surface potential lowers and an area where the surface potential does not lower as the light energy increases. The boundary line between the two areas is a boundary point, and the following measurement is performed with a lower light quantity.
As shown in FIG. 3, the variation of the irradiated part potential is measured when the irradiation-development time is changed by the measurer disclosed in Published Unexamined Japanese Patent Application No. 2000-275872. Then, as shown in FIG. 4, when the relationship between the irradiation-development time and the irradiated part potential is plotted, a folding point can be found. The irradiation-development at the folding point is defined as a transit time in the present invention. Therefore, the relationships among the irradiation-development time, the irradiated part potential and the transit time, i.e., the time responsiveness of the surface potential light attenuation of an electrophotographic photoreceptor can exactly be known. The transit time depends on the surface potential and thickness of a photoreceptor before irradiation with writing light, in other words, on the electrical field intensity applied to a photoreceptor. Therefore, when the transit time is measured, a photoreceptor having the same compositions and thickness as those of a photoreceptor actually used is needed. The surface potential of a photoreceptor before irradiation with writing light needs to be equivalent to an unirradiated surface potential of an image forming apparatus in which the photoreceptor is used.
A method of controlling the transit time of a photoreceptor will be explained in detail when a photoreceptor is explained. The present inventors analyze the transit time of a typical negatively-charged multilayer photoreceptor including a substrate, and an intermediate layer, a CGL and a CTL in this order on the substrate. As a result, the transit time reflects the transportability of a photocarrier generated in the CGL, and eventually reflects the positive-hole transportability in the CTL mostly. In order to effectively control the transit time, the formulation of the CTL proves to be essential.
The irradiation-development time is defined as a time for a given point on the photoreceptor to transport from a position right in front of the irradiator to a position right in front of the image developer. More specifically, as FIG. 1 shows, a time for a given point on the photoreceptor to transport from a position (A) right in front of the irradiator to a position (B) right in front of the image developer while the photoreceptor rotates in the direction of a dashed arrow. The position (A) is a center of writing light (beam) emitting from a writing light source to the center of a photoreceptor, and an intersecting point between the writing light and the surface of the photoreceptor. The position (B) can be said the center of a developing nip, and when a developing sleeve having the shape of a rod is used as FIG. 1 shows, can be said a position where the developing sleeve and the surface of the photoreceptor come closest to each other. Therefore, the irradiation-development time is a time (sec) from dividing a length (mm) of a circular arc from the position (A) to the position (B) by a linear speed (mm/sec) of the photoreceptor.
Thus, the relationship between the transit time and the irradiation-development time is clarified.
In the compact image forming apparatus (the diameter of the photoreceptor is not greater than 40 mm), producing high-resolution images (not less than 1,200 dpi) at a high speed (not less than 50 pieces/min), the photoreceptor needs to finish light attenuation in the irradiation-development time. When writing light is irradiated thereto in a short time as a laser beam after the photoreceptor is charged, the surface potential of the photoreceptor gradually attenuates as time passes. The potential largely attenuates for a specific time, but after the specific time passes, the potential scarcely attenuates. The specific time can be thought a transit time during which most of the photocarriers generated in the photoreceptor pass over a photosensitive layer thereof.
The time is expected to depend on the carrier generation and carrier transport time in the photoreceptor, the relationship between the process conditions and transit time is not clarified when using a tandem full-color image forming apparatus.
When the irradiator cannot follow, the irradiance level to the photoreceptor lowers, resulting in deterioration of image density in negative-positive development and deterioration of color balance in a tandem full-color image forming apparatus. Therefore, the writing image resolution is decreased.
When the transit time is longer than the irradiation-development time, the irradiated part of a photoreceptor reaches the development part while a photocarrier generated in a photosensitive layer of the photoreceptor is still being transported Therefore, (i) the surface potential of the photoreceptor does not sufficiently lower and development potential is not fully obtained, resulting in deterioration of image density in negative-positive development. (ii) Should the development potential be obtained, the surface potential lowers after passing the development part and the electrostatic adherence of a toner to the irradiated part of the photoreceptor lowers in negative-positive development, resulting in deterioration of dot image resolution or toner scattering when transferred. (iii) Further, when the photoreceptor forms a second image after forming a first image, a carrier generated late inside slightly lowers the irradiated part potential of the first image. Therefore, halftone potentials differ from each other, resulting in production of abnormal images such as a ghost (a residual image) in a monochrome image forming apparatus and deterioration of color reproducibility in a full-color image forming apparatus producing many halftone images.
Because of these reasons, a need exists for a compact image forming apparatus producing high-quality images at high speed.